The present invention relates to a process for the removal of pyritic sulfur from coal.
Most coals which are available for use in this country contain a high concentration of sulfur which must be reduced to a minimum level in order that these coals may be combusted without the emission of objectionable quantities of sulfur oxides into the atmosphere. The sulfur is generally present in coal in the form of sulfate sulfur, organic sulfur and pyritic sulfur. The organic sulfur is chemically bonded within the organic molecular framework of the coal, while the pyritic sulfur consists of sulfur in the form of iron pyrite, which is disseminated as a separate mineral phase throughout the body of the coal. In general, sulfate sulfur constitutes a minor fraction of the sulfur content in coals, i.e., less than about 0.2 weight % of the coal. The organic sulfur and pyritic sulfur constitutes the major fraction of sulfur which is present in coal and together they constitute up to about 5-8 weight % of the coal. The pyritic sulfur constitutes between 40 and 60 percent of the total sulfur content in the coal. Therefore, the removal of the pyritic sulfur alone can significantly reduce the sulfur content and, therefore, the sulfur emissions which occur upon combustion of coal.
Because the organic sulfur is an integral part of the chemical structure of the coal, it is impossible to remove this sulfur without severely disrupting the chemical bonding which occurs within the structure of the coal. As a result, processes which remove substantial fractions of the organic sulfur are characterized by extreme process conditions, e.g., pressure, temperature, etc. They are, therefore, expensive and require the input of large quantities of energy. Moreover, the product which results from such processes has distinctly different properties from the starting coal as relate to structure, chemical composition, grindability and combustibility.
The pyritic sulfur, on the other hand, exists as a distinct phase within the body of the coal. It is, therefore, possible to liberate the pyrite from the coal physically, and by means of selective physical or chemical techniques to remove the liberated pyrite from the coal, without altering in any significant way the properties of the coal.
Heretofore, a variety of physical and mechanical methods have been employed to remove pyrite from coal. These include heavy media separation, jig tables, selective agglomeration and floatation. In general, however, these techniques result in substantial removal of pyritic sulfur coupled with a significant recovery of cleaned coal only when the size of the pyrite particles disseminated throughout the coal is greater than about two millimeters. Because the size of pyrite particles within the vast majority of coals is much smaller than that which can be effectively removed by the above-mentioned techniques, these techniques have found only limited application. In order to remove this finely-divided pyrite from the coal after it has been liberated by mechanical means, it is generally necessary to employ chemical methods.
It is generally known that pyritic sulfur can be removed from coal by chemical oxidization to a species which is soluble in water. Such processes are described, for example, in U.S. Pat. No. 3,768,988 to Meyers and U.S. Pat. No. 3,960,513 to Agarwal. Since these processes require the use of both elevated temperatures and pressures, however, they have not resulted in cost-effective methods for removing pyrite from coal.
Microbiological processes for leaching pyrite as well as other mineral sulfides from inorganic ore bodies under conditions near ambient are also well known in the art. For instance, U.S. Pat. No. 2,829,964 to Zimmerly discloses a cyclic leaching process for extracting metallic constituents from inorganic metallurgical materials, using a leaching medium which is cyclically regenerated by the action of iron oxidizing bacteria. Such processes have proven successful in leaching chalcopyrite (CuFeS.sub.2) to produce a product containing substantially pure copper.
Attempts have been made in the past to utilize similar microbiological processes for leaching pyrite from coal. These attempts have not proven altogether successful for reasons that have not heretofore been completely understood. Probably the earliest work on microbiological leaching of pyrite from coal was conducted by Silverman et al. and is reported in an article by these workers, appearing in FUEL, Vol. 42, published by I.P.C. Science & Technology Press, Surry, England, 1963. In this earlier work, experiments were conducted using small volumes of a slurry containing 2.5 weight % coal in flasks. The pH of the slurry was adjusted to an initial value of between about 2.5 and 3.5. The slurry was then inoculated with approximately 5.times.10.sup.9 cell/ml. of a resting cell suspension of an iron and sulfur oxidizing microorganism i.e. Thiobacillus ferrooxidans. This cell concentration was equivalent to a ratio of 2.times.10.sup.12 -2.times.10.sup.13 cells per gram of pyrite depending on the particular coal. The flasks were shaken at room temperature to agitate the slurry. It was reported in this article that the leaching action effectively stopped after a period of approximately four days. During this period, it was determined that between 45 and 55 percent of the pyrite had been removed. Other results reported in the same article are highly variable and indicate that depending on the particular coal being treated, approximately between zero and 76 percent of the pyritic sulfur was removed. Although the early work of Silverman, et al. did attempt to follow the teachings of the prior art relating to microbiological leaching of inorganic sulfide minerals, the results of this work surprisingly indicate that the prior art processes do not successfully lend themselves to the removal of pyrite from coal. The exact reasons for the apparent ineffectiveness of prior art processes for mineral leaching when applied to coal are not completely understood. However, it is believed that this result is attributable in part to the inherent differences between the organic coal and the inorganic mineral rock material exposed to the leaching medium. For example, it is postulated that relatively rapid and complete leaching of pyrite from coal requires an actively growing population of microorganisms and that the conditions for maintaining such a population in the presence of organic coal material are different than those which have been employed in prior art process for leaching inorganic mineral material.